This invention relates to a television stereo adapter and, more particularly, to such an adapter which is physically separate from the conventional television receiver with which it is used to detect and decode stereophonic audio signals that may be transmitted but which are not otherwise detectable by the television receiver. Advantageously, no physical or electrical (i.e. no wired) connections to the television receiver are needed for the successful operation of the stereo adapter. Optionally, however, enhanced results may be attained by connecting the adapter to the usual multiplex output (or MPX) jack of the television receiver.
Recently, the transmission of stereophonic audio signals for television broadcasting has been approved. The format for such stereophonic audio broadcasting is the so-called Zenith system described in, for example, Audio Magazine, June 1984, pages 47-51, and also in Engineering Report GL-84091, National Semiconductor, September 1984, "A Stereo TV Decoder for Zenith System". As described in the aforementioned publications, the Zenith system of stereophonic audio broadcasting includes a monophonic (L+R) component whose base band ranges from 0 to 15 KHz, this monophonic component being frequency modulated onto the usual audio (or sound) frequency carrier f.sub.S that normally accompanies composite television signals, as is conventional, a stereophonic (L-R) component that is amplitude modulated onto a subcarrier of frequency 2f.sub.H (wherein f.sub.H is the usual horizontal synchronizing frequency of 15.734 KHz), the amplitude modulated stereo component being of the double sideband suppressed carrier type, a pilot signal of frequency f.sub.H, a so-called "second audio program", referred to by those of ordinary skill in the art as SAP, frequency modulated onto a subcarrier whose frequency is 5f.sub.H, and an auxiliary channel, referred to as a professional channel in which voice or data information may be transmitted, this auxiliary channel being frequency modulated onto a subcarrier of frequency 6.5 f.sub.H. The monophonic component, the amplitude modulated double sideband suppressed carrier stereophonic component, the pilot signal, the second audio program and the auxiliary channel all frequency modulate the usual audio carrier f.sub.S. Since there is substantially no change in the monophonic (L+R) component, and since this monophonic component serves to frequency modulate the usual audio carrier f.sub.S, conventional television receivers that are not equipped to detect or decode the multiplexed stereo signals nevertheless can detect and demodulate the monophonic (L+R) component in the conventional manner. However, those television receivers that are equipped with a stereo detector/decoder compatible with the Zenith system will detect not only the monophonic (L+R) component but also the stereophonic (L-R) component and, by conventional matrix decoding, derive from the monophonic and stereophonic components separate left-channel (L) and right-channel (R) audio signals. Hence, separate left-channel and right-channel loudspeakers may be driven to reproduce stereophonic sounds.
The aforementioned stereo-equipped television receivers also may detect and decode the second audio program which, typically, may provide bi-lingual (or translational) information associated with the broadcasted television program. It also is contemplated that the information transmitted in the auxiliary channel will be detected and utilized either by the television receiver or by other means connected to the television receiver and supplied with the detected information.
To improve and enhance the signal-to-noise ratio of the stereophonic (L-R) component, a noise reduction companding system is used. More particularly, the stereophonic component is subjected to noise reduction compression prior to transmission and to expansion upon reception. The particular noise reduction compression technique that has been adopted for use with the Zenith system is the so-called dbx companding system which has been developed by DBX Inc. of Waltham, Mass. It is, of course, expected that the stereo detector/decoder included in stereo-compatible television receivers will be provided with a noise reduction expansion circuit that is compatible with the dbx compression circuit.
Although the monophonic (L+R) component of the transmitted stereo audio signals still will be detected by conventional television receivers that are not equipped with stereo detectors/decoders, stereophonic sound reproduction, or the sensation (or simulation) of stereophonic sound will not be reproduced. It has been proposed, heretofore, that such conventional television receivers be modified (or retrofitted) in order to detect and reproduce stereophonic sound from TV stereo transmissions. Alternatively, separate receivers capable of receiving the audio portion of television transmissions, that is, the audio carrier f.sub.S, and detecting and decoding the stereo signals therefrom, would have to be purchased.
The aforementioned proposals of retrofitting conventional television receivers to include a stereo detector/decoder or purchasing special TV stereo sound receivers not only are expensive but, in many instances, will not be preferred by the several million consumers who now own conventional television receivers that are not compatible with stereo TV transmissions. Consequently, it is believed that, once stereo TV transmissions become more prevalent, there will be a need for a relatively simple adapter that can be used without significant modifications to conventional television receivers. Advantageously, such a stereo TV adapter should be physically and electrically independent of the television receiver, thus requiring no internal wiring or soldering which otherwise would be necessary for the adapter to be compatible and usable with the television receiver. In particular, it is desirable to permit the adapter to pick up the received audio portion of the television program without providing a wired circuit connection between the adapter and the conventional components of the television receiver.